Study on Geotechnical aspects related to rehabilitation of deteriorated sewer pipelines

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In Japan, many old sewer pipelines installed in urban areas were severely deteriorated to decrease their functions and increase the possibility of ground depression. Sewer pipes are not pressure pipes. If the old pipes are deteriorated, therefore, soil particles tend to flow into the pipes with water, easily developing voids in the ground to cause ground depression. However, mechanism and actual condition of void development in the ground have not been clarified yet. Also, since excavation for replacing the deteriorated pipes by new pipes is difficult in urban areas, no-dig rehabilitation methods using plastic inner liners have been developed and are increasingly being used in the world. The plastic inner liners have long lives, but their rigidity decreases according to the elapsed time due to relaxation. On the contrary, the rigidity of backfill soils around the pipes increases according to the elapsed time. Therefore, mechanical behavior of buried pipes that is governed by relative rigidity between soil and pipe will be changed according to the elapsed time. However, this problem has not been studied hitherto.This study was carried out to clarify mechanism of void development in the ground around deteriorated old sewer pipelines and to investigate long-term change in soil-structure interaction. The main conclusions can be summarized as follows:1) Inflow of soil through opening at joints of the pipeline develop the voids in the ground. The location of the voids is at the upper part of the pipe, but not at the top of the pipe. Cracks along the long axis of the pipe do not produce the voids in the ground.2) Apparent rigidity of backfill soils decrease due to repeated vehicle loading, resulting in increase in deflection of plastic pipes. The relaxation of plastic pipes during the elapsed time remarkably strengthens the deflection of plastic pipes.